1. Field of the Invention
The present invention relates to a method and apparatus for validating paper currency, particularly United States one, two and five dollar bills, and more particularly to such a method and apparatus in which the authenticity and denomination of paper currency is identified by sensing the characteristics of a piece of currency along a predetermined scan line.
2. Description of the Prior Art
A number of devices have been proposed which identify and distinguish between various denominations of U.S. paper currency or "bills", but none of these devices has been completely satisfactory.
Genuine U.S. paper currency contains a variety of printed indicia which may be used to identify the currency as authentic, and also to distinguish between authentic currency of various denominations.
One indication of authenticity is the fact that certain areas on a U.S. bill are printed with ink with magnetic properties. For example, the portrait which appears in the center of every U.S. bill is, in a genuine bill, printed entirely with magnetic ink. The fanciful engraving which forms the printed border of each U.S. bill is likewise composed entirely of magnetic ink, as are the large capital letters or large numerals which appear to the right of the portrait and which identify the denomination of the bill (i.e., "ONE", "TWO", "FIVE", etc.). In contrast, the green Treasury Department seal which underlies the denomination identifying letters or numerals to the right of the portrait, as well as the black Federal Reserve Bank seal which appears to the left of the portrait, are both printed in non-magnetic ink.
Each denomination U.S. bill is likewise characterized by the distance between the grid lines which comprise the background of the portrait field. In one dollar bills, for example, the space between vertical grid lines is equal to 0.008 inches. For two and five dollar bills, the grid line space is equal to 0.010 inches and 0.011 inches, respectively.
Prior art currency validators have been proposed which identify authentic U.S. bills and distinguish between bills of various denominations by measuring the average spacing between the vertical grid lines in the portrait areas of the bills. One such device is disclosed in U.S. Pat. No. 4,349,111 to Shah et al.
Identification of bills based on average grid line spacing is likely to lead to failures to distinguish between bills having relatively small differences in grid spacing. For example, certain commercial bill validators utilizing the average spacing technique cannot be used with both two dollar and five dollar bills, because the average grid line spacings are too similar.
Another problem with various prior art validators is that they may accept high denomination bills as valid lower denomination bills.
Many prior art currency validators require that the tested bill be inserted into the validator in a specific orientation (e.g., Federal Reserve seal first). Such devices result in authentic bills being rejected merely because of improper orientation. It is therefore desirable to provide a currency validator which is operationally insensitive to bill orientation.
Many of the prior art currency validators require careful regulation of the speed at which the bill is scanned for information. In such validators, even a slight variation in scanning speed, such as that resulting from an instantaneous drop in power line voltage, can cause authentic bills to be rejected and produce inaccuracies in the identification of bill denomination. It is therefore desirable to provide a currency validator which is insensitive to the speed at which a bill is scanned.
In order to avoid some of the problems of speed regulation, some prior art validators, such as disclosed in U.S. Pat. No. 4,464,787 to Fish et al, employ detectors at fixed positions to positively identify the position of the bill and thereby ascertain the bill area being tested. These validators, however, generally require a testing channel at least as long as the bill being tested.